This invention relates to so-called "smart" circuit interrupters employing digital electronic trip units. One such smart circuit interrupter is described within U.S. Pat. No. 4,589,052. Such digital trip units are capable of acting on a number of programmed setpoint trip parameters to interrupt the circuit being protected in a timely manner. One such digital trip unit which allows for user interaction and which provides capability both to select trip parameters and to view the selected trip parameters is found within U.S. Pat. No. 4,672,501.
For purposes of reducing cost while maintaining accuracy and functionality of the digital trip units, microprocessors are being employed to gather information, process the information, and provide a means of monitoring the information using associated digital circuitry. The use of such microprocessors allows more protection functions to be contained in smaller-sized enclosures.
U.S. Pat. No. 4,870,531 entitled "Circuit Breaker with Removable Display and Keypad" describes a user-interface unit that contains display capability for a multiplicity of circuit interrupter options by means of software located within the trip unit microprocessor while only displaying those parameters for which the digital trip unit is configured.
U.S. Pat. Nos. 4,945,443 and 4,991,042 both describe digital circuit interrupters employing user-accessible keypads and displays for both metering and circuit interruption. The intelligence contained within such "smart" circuit interrupters lies within the digital processor contained within the electronic trip circuit, usually housed within the circuit interrupter cover. However, such digital processors can be susceptible to powerful localized radio frequency interference signals hereafter "RFI" emanating from portable communication transmitters and telephones as well as localized electromagnetic interference hereafter "EMI" caused by high inductive switching circuits. RFI and EMI signals reaching the trip unit by parasitic transmission over the electric power distribution cables are readily filtered out from the trip unit circuit by conventional electronic filters. RFI and EMI signals that are transmitted to the trip unit circuit through the atmosphere can result in so-called "nuisance" tripping whereby the trip unit circuit becomes energized to a trip mode and, in turn, articulates the circuit breaker operating mechanism to interrupt circuit current during quiescent circuit current conditions. The shutdown of associated electrical equipment used within manufacturing facilities due to such nuisance tripping often results in unnecessary economic loss.
To delete such RFI and EMI signals from interfering with state-of-the-art electronic devices, a filter is generally interposed between the electronic devices and the possible sources of the RFI and EMI signals. U.S. Pat. No. 4,814,566 describes a pushbutton keyboard assembly employing EMI and RFI-shielded switch modules. U.S. Pat. No. 5,068,493 describes a gasket shielding for electronic equipment which contains embedded metal flakes. U.S. Pat. No. 4,884,171 teaches the use of EMI gaskets for shielding aircraft engine testers. U.S. Pat. No. 5,041,707 discloses a metallic cover plate used to provide EMI shielding to electric switches used on military aircraft.
One arrangement for protecting prior art digital circuit interrupting components from RFI and EMI signals comprises the application of electrically conductive paint directly onto the plastic housing that contains the sensitive electronic components during the circuit interrupter assembly. The direct application of conductive paint to the cover that houses the smart circuit interrupter components, however, is not feasible because of the risk of contacting the circuit interrupter components with the conductive paint. The extensive masking and venting techniques employed with the prior art digital circuit interrupter assembly processes are not feasible for the smart circuit interrupters due to the automated assembly of the digital electronic components as well as the compact arrangement of the electronic components within the circuit interrupter enclosure.
Accordingly, one purpose of this invention is to provide an inexpensive RFI and EMI filter that can be inserted within the smart circuit interrupter enclosure in an automated assembly process without requiring masking of the enclosure or venting of the environment surrounding the assembly equipment.